Method and apparatus for removing entrained dust particles from a gaseous stream

ABSTRACT

An apparatus for removing entrained solid particles from a gaseous stream using a wet-type dust collection system embodying the venturi principle. Water droplets are introduced into the particle laden gaseous stream upstream of the venturi throat by means of rotating drums having a lower surface portion immersed in a water trough. The rotating drums pick up water on their surfaces and propel small droplets tangentially outward in the form of a spray. The venturi throat is defined in part by a pair of rollers journaled for free rotation and having parallel axes spaced so that adjacent surfaces of the rollers define between them a restriction. The force of surface friction between the roller surfaces and the moving stream causes free rotation of the rollers thus reducing frictional resistance to stream flow and distributing erosion over a relatively large area.

United States Patent Deacon 1 July 11, 1972 54] METHOD AND APPARATUS FOR3,495,813 2/1970 Marenghi et al...........................5$/86 REMOVINGENT U 3,427,006 2/1969 Oiestad ...26l/DlG. s4 A 3,2l2.23$ lO/l965Markant ..$$/89 STREAM Pn'mar Examiner-Charles N. Hart [72] Inventor:James S. Deacon, 4492 Camellia Lane, Y Greene Howe N rthOl t d,Oh 440700 ms e IO CT 22 Filed: Dec. 9, i970 I An appaiatus for removingentrained solid particles from a [2|] Appl. No: 96, gaseous stream usinga wet-type dust collection system embodying the venturi principle. Waterdroplets are introduced 52] Us 0 55/227 55/230 55/257 into the particleladen gaseous stream upstream of the venturi 26d 26m); 54, them y meansofrotafing d having a low surface p [5 1] IL Cl 80M n tion immersed in awater trough. The rotating drums pick up [58, was. m11111111111111:1315555353? 91 227 weeee en eheee eeefeeee eee eeeeeleeeelee eeeeeeeelly 55/228, 229, 230-232, 257; 26l/DlG. 9, DIG. 54, 62,83, 84, 92, I15, H8

outward in the form of a spray. The venturi throat is defined in part bya pair of rollers journaled for free rotation and having parallel axesspaced so that adjacent surfaces of the rollers define between them arestriction. The force of surface friction between the roller surfacesand the moving stream causes free rotation of the rollers thus reducingfrictional resistance to stream flow and distributing erosion over arelatively large area.

6Claims,3Drawingflgures PA'TENT'EDJUL 1 1 I872 SHEEI 2 OF 3 INVENTOR.

JAMES S. DEACON M0604 Qaeene Z Jewell ATTORNEYS PKTENTEDJUL 1 1 m2 SHEET30F 3 INVENTOR.

JAMES S. DEACON 440604;, Gwene a /awed ATTORNEYS METHOD AND APPARATUSFOR REMOVING ENTRAINED DUST PARTICLES FROM A GASEOUS STREAM BACKGROUNDOF THE INVENTION This invention relates to wet-type dust scrubbers suchas are used for purifying dust laden gases and especially to aventuritype dust scrubber that uses impingement of the dust particles onsmall water droplets to capture or agglomerate the dust. The venturieffect assists in the atomization of the water droplets and causesbeneficial turbulence in the gaseous stream.

The agglomeration of the dust particles by the water droplets resultsfrom at least two factors. One factor is the impingement of the fastmoving dust particles on the finely atomized water droplets which areinitially (i.e. just after injection) moving at a much slower relativevelocity in the stream. The high relative velocity of a dust particle asit strikes a water droplet results in the "capture" or agglomeration ofthe particle by the water.

The second factor is the formation of condensation on the dust particlesas they pass through the venturi throat due to the reduction in fluidpressure. This presure drop combined with the vapor-saturated conditionof the gas achieved by in jecting the water causes condensation to formon the particles due to the Joule-Thompson efi'ect with resultingagglomeration of the particles.

One of the most important factors affecting the operation ofventuri-type dust collectors is the effectiveness of the spraying ofwater into the moving stream. According to most current practices spraynozzles are used to inject the water usually at right angles to thestream flow. This system has the disadvantage that contaminatedscrubbing water cannot be reused even if it is partly clarified. Anyattempt to reuse scrubbing water containing only a small volume ofcontaminates would result in plugging up of the nozzle orifices.

One method of introducing water particles in a way that permits reuse ofpartially contaminated scrubbing water is that of flowing the water overa weir using continuous pressure that causes water to overflow an edgeof a trough and fall into the path of the dust laden gaseous stream. Thecontaminated water may be clarified using for example the method andapparatus of my copending US. Pat. application Ser. No. 831,180 filedJune 6, I969. The clarified water may then be recirculated back throughthe system. While this waterfalr method has certain advantages it failsto achieve optimum dis integration of water droplets into a fineatomized spray which is necessary in order to achieve maximumagglomeration of dust particles and resulting high efficiency. For thepurpose of this description the word efficiency means that portion ofthe total volume of dust entrained in the gaseous stream, that isremoved by the scrubbing system, the removed portion being expressed asa percentage of the total volume of dust.

Another problem occurring in prior art venturi-type dust scrubbers isthe erosion of the surfaces forming the venturi throat. Due to the highstream velocity occurring in the throat and the constriction of thestream, the particles, which are often quite abrasive, strike the throatsurface and erode the material. Over a period of time the erosionresults in a variation in the cross sectional dimensions of the venturithroat and changes the throat configuration. This results in a reductionin the effectiveness of the venturi in agglomerating the dust particlesand thus reduces the overall efficiency of the system.

Another problem is that of providing a venturi-type dust scrubber thatis sufficiently flexible to handle wide variations in stream velocity.The optimum configuration or geometry of the venturi throat will dependof course upon the stream velocity and where the velocity isconsiderably different from that for which the throat was designed theoptimum efficiency cannot be achieved.

The apparatus of the present invention however satisfies thedifficulties indicated above and affords other features and advantagesheretofore not obtainable.

SUMMARY OF THE INVENTION It is among the objects of the invention toprovide a wettype dust collection system using a venturi-type dustscrubber that is capable of operation for long periods of time withoutcleaning and repair.

Another object is to provide a venturi-type dust scrubber wherein theerosion caused by abrasive particles striking the surfaces of theventuri throat is substantially reduced.

Still another object is to provide improved means for introducing aspray of water into a dust laden gaseous stream entering a dust scrubberin such a way that the condition of the water and the amount ofcontaminates contained therein will not effect the overall operation.

A further object is to provide a wet-type dust collector with a venturithroat wherein the configuration and dimensions of the throat can beadjusted to provide optimum geometry under varying conditions of streamvelocity.

These and other objects are accomplished by means of an improved throatwet-type dust collector construction wherein a venturi throat isinterposed in the path of a gaseous stream having entrained dustparticles and wherein an inlet and outlet is provided for the stream.According to the invention water droplets are introduced into the pathof the stream by means of one or more water troughs and one or moredriven spray drums, each having a portion of its surface immersed belowthe surface of the water in the respective trough so that by rotatingthe drum, water that clings to its surface is propelled tangentiallyinto the path of the stream in the form of small droplets that are mixedinto and carried by the stream. A pair of cylindrical rollers withspaced parallel axes,journaled in the unit are used to define theventuri throat. The rollers have portions of their respective surfacespositioned on opposite sides of the stream to define therebetween arestriction. Since the rollers are journaled for free rotation thefrictional force between the surface of the rollers and the gaseousstream causes rotation of the rollers so that any erosion of the throatforming surfaces is distributed uniformly around the surfaces of therollers. Also resistance to stream flow resulting from surface frictionis reduced due to the relatively small differential between the surfacevelocity of the rollers and the velocity of the particles in the stream.

Also associated with the unit is a means for removing the agglomerateddust particles and water droplets from the stream after the streamleaves the scrubber.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is an elevational view insomewhat diagrammatic form illustrating a foundry cupola installationhaving a wettype dust scrubber using a venturi principle for removingdust particles from the foundry gases;

FIG. 2 is a fragmentary elevational view on an enlarged scale of thedust collector of FIG. 1 with parts broken away and shown in section forthe purpose of illustration;

FIG. 3 is an elevational view of the dust scrubber of FIG. 2 with partsbroken away at the line 3-3 of FIG. 2, and shown in section for thepurpose of illustration.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring more particularly tothe drawings and initially to FIG. I there is shown a dust scrubber Aadapted to agglomerate particle laden gases from a foundry cupola B. Thewater droplets and agglomerated dust particles that are present in thegaseous stream leaving the dust scrubber A are separated from the streamin an air-water separator C and a portion of he contaminated water issubsequently reused in the scrubber A after clarification. Air is forcedinto the cupola B by a blower I0 and the charge is introduced through acharging door opening 11. The resulting gaseous combustion products andentrained dust particles flow into a dry cap 12 with a direct outlet 13to atmosphere and also a bypass conduit 14 that delivers the gaseousstream to the scrubber A.

Under normal operating conditions exit of gases through the outlet 13 isblocked by means of a valve unit that includes a valve plate 16 to closethe outlet 13. The plate 16 is controlled by means of a crank arm 17operated by the piston rod 18 of an air cylinder 19. Normally the valveplate 16 will be opened only when the rate of gas flow from the cupola Bexceeds the flow rate capacity of the scrubber A. Under normal operatingconditions all of the particle laden gases will be passed to thescrubber A through the conduit 14.

The gases exit the scrubber A through an exit conduit 21 that deliversthe gaseous product containing agglomerated dust particles and waterdroplets to the separator C. The separator C has an exhaust stack 22which exhausts the purifled gases to the atmosphere. within theseparator C the gases take up a helical path of travel and the resultingcentrifugal action drives the agglomerated dust particles and waterdroplets to the wall of the separator casing where they drip downward toa sludge drain line 26. From the line 26 the contaminated water orsludge is delivered to a clarifier 24 shown diagrammatically in FIG. 1.The clarifier may be for example one of the type disclosed in mycopending US. Pat. application Ser. No. 83 L180 filed June 6, l969. Fromthe clarifier 24 the clean or clarified water is pumped through a line25 for recirculation back to the scrubber A. That portion of thecontaminated water that is left in the form of sludge is delivered to asludge bed or dewatering unit 27 (shown diagrammatically in FIG. 1) orother means for disposal. [t is expected under typical circumstancesthat about 90 per cent of the scrubbing water may be returned orrecirculated to the dust scrubber A while the remaining 10 per cent isremoved. Accordingly additional water must be continuously supplied andthis is accomplished through the water supply line 28.

The dust scrubber A will be best understood with reference to F 165. 2and 3 and comprises an enclosure 30 formed for example of steel plate.The enclosure 30 has a generally rectangular or box-like form with afront wall 31, a rear wall 32 and end walls 33 and 34. Located withinthe enclosure 30 are two venturi throat rollers 35 and 36 mounted withtheir respective axes parallel to one another and perpendicular to thefront and rear walls 31 and 32 with their surfaces defining arestriction zone therebetween that serves as a part of the venturithroat for the scrubber A. The rollers 35 and 36 are sealed relative tothe front and rear walls 31 and 32 of the enclosure 30 by means ofsealing rings 37 and are journaled for free rotation on non-rotatingaxles 38 and 39 respectively, by means of ball bearing units 40. It isparticularly important that the venturi throat rollers 35 and 36 bemounted on their respective axles 38 and 39 to provide a minimum ofresistance to their free rotation.

The rollers 35 and 36 are preferably formed of stainless steel or othercorrosion resistant metal and preferably have surfaces with a lowcoefficient of friction. For example the metallic surfaces may betreated such as by an electroplating process to provide a low friction,corrosion resistant surface or may in fact have their surfaces coatedwith a low friction hydrocarbon material such as TEFLON."

The axles 38 and 39 each have eccentric stub shafts 41 and 42 extendingfrom their opposite ends, the shafts of each axle being coaxial with oneanother. The stub shafts 41 and 42 are each journaled in a bushing 43mounted in each of two bearing blocks 44 and 45 respectively. Thebearing blocks 44 and 45 are mounted on the front wall 31 and ear wall32 respectively of the enclosure 30. The eccentric stub shafts 41 ofeach of the venturi throat rollers 35 and 36 have a longer axial lengththan the stub shafts 42 at the opposite ends, and have an annular sleeve46 keyed thereto. Welded to each of the sleeves 46 is one of twopositioning levers 47 and 48 which may be used to turn the stub shafts41 and 42.

It will be seen that turning of the eccentric stub shafts 41 and 42about their axis causes a change in the position of the axes of therespective venturi rollers 35 and 36 and thus changes the spacingbetween the venturi rollers 35 and 36 to vary the size of the venturithroat.

in order to facilitate the mounting and removal of the venturi rollers35 and 36 and their associated structure the front and rear walls 31 and32 of the enclosure 30 are provided with oval shaped cut outs. These areclosed by closure plates 49 and 50, the plates being secured to thefront wall 31 and rear wall 32 respectively by means of perimetricflanges 51 and 52 that are bolted to the respective walls. The bearingblocks 44 and 45 are welded at the appropriate positions to the closureplates 49 and 50 so that the rollers 35 and 36 are integral with theclosure plates and their respective flanges 51 and 52.

As indicated above the spacing between the surfaces of the venturirollers 35 and 36 can be varied by means of the positioning levers 47and 48 respectively. in order to secure or fix the positioning levers 47and 48 at a desired roller spacing a pair of semi-circular locking bars53 and 54, one for each positioning lever 47 and 48, are attached to thefront wall 31 of the scrubber enclosure 30 by means of spacer brackets55. A number of positioning holes 56 are formed in the locking bars 53and 54 at spaced locations in order that a number of different spacingselections can be made. Each of the positioning levers 47 and 48 has ahole 57 that registers with any of the holes 56 so that the respectivelevers may be secured by means of a locking pin 58.

The venturi throat is also defined in part by means of upper and lowerthroat plates 60 and 61 respectively which extend from front to rearparallel to the axes of the respective rollers 35 and 36. The upperthroat plates 60 are so arranged as to cause the air stream enteringthrough the inlet 14 to converge to the space defined by the adjacentroller surfaces while the lower throat plates 61 cause the stream todiverge toward the exit conduit 2 1.

The throat plates 60 and 61 are pivotally mounted by means of hinge pins62 and 63 respectively in order that the positions of the plates 60 and61 may be adjusted. The throat plates 60 and 61 must be capable ofpivotal movement about their respective axes so as to accommodatedifferent positions of adjustment of the venturi rollers 35 and 36. Theupper throat plates 60 rest at their lower edges and by virtue of theirown weight against the surfaces of the respective rollers 35 and 36 andthe rest position is dependent upon the particular adjustment of therespective rollers. Other means for mounting and adjusting the upperthroat plates 60 may of course be employed in order to eliminate anycontact between the bottom edges of the plates 60 and the surfaces ofthe respective rollers 32 and 33.

The lower throat plates 61 are held in a generally upright position bymeans of rods 64 pivotally connected at one end to a bracket 65 on theirrespective plate 61 and which extend outwardly through the end walls 33and 34 of the enclosure 30. The outwardly extending ends of the rods 64have threads formed thereon to receive tensioning nuts 66 that retainsprings 67 between the inner faces of the nuts and the surface of therespective end walls 33 and 34.

Located in the upper portion of the scrubber enclosure 30 adjacent theend walls 33 and 34 are two water troughs 71 and 72 which contain asupply of water with a level almost to the top of the walls of thetroughs 71 and 72. The water level is maintained constant by means of afloat valve assembly 73 of conventional type and which forms no part ofthe invention.

Operatively associated with each of the troughs 71 and 72 is one of twospray drums 74 and 75 that are secured to shafts 76 and 77 by means oflock screws 78. The ends of the shafts 76 and 77 extend through thefront wall 31 and rear wall 32 of the enclosure 30 and are journaled inbearing blocks 79 and 80. The ends of the shafts 76 and 77 that extendthrough the front wall 31 of the enclosure 30 each have a pulley 81keyed thereto, each pulley 81 being driven by a belt 82 which in turn isdriven by a motor pulley 83 mounted on the output shaft 84 of a drivemotor 85. Each of the drive motors 85 is mounted on a motor bracket 86bolted to the enclosure 30.

The drums 74 and 75 are so mounted that the bottom surface portionsextend into and below the surface of the water in the troughs 71 and 72.Thus as the drums are driven, water that clings to the surface of thedrums due to surface tension is quickly propelled tangentially from thesurface by centrifugal force to cause a continuous spray. The spraypattern is determined by the speed of the drums and will normally beadjusted to provide approximately the pattern indicated in FIG. 2 sothat an optimum spray is produced in the path of the incoming gaseousstream,

The surfaces of the drums are preferably provided with a treatment orlayer of material which resists formation of scale and other depositsduring operation. It should be noted however that the formation of scaleand other deposits will not greatly hamper the spraying operation of thedrums since as long as the surfaces thereof extend below the surface ofthe water in the troughs, they will continue to pick up and spray thewater into the path of the incoming gaseous stream. Although depositsmay collect in the bottom of the troughs 71 and 72 this again will notaffect the operation although it may be desirable to clean the troughsat certain time intervals to remove scale deposits etc. It will beapparent that scrubbing water may be easily reused with this systemsince the presence of contaminates in the water will not affect thespraying operation.

OPERATION When the foundry cupola B is to be put into operation thescrubber A is initially readied by supplying enough water to the troughs71 and 72 to fill them to the level at which the water is shut off bythe float valves 73. The motors 85 are then started and adjusted inspeed until an optimum water spray pattern is provided in the path ofthe incoming gaseous stream. Then using the positioning levers 47 and48, the venturi throat rollers 35 and 36 are adjusted to an approximateposition to satisfy the expected velocity conditions etc. resulting fromthe operation of the foundry cupola B. When the cupola is put intooperation the air cylinder 19 will close the valve plate 16 and all ofthe gases will be directed from the dry cap 12 through the conduit 14into the scrubber unit A carrying the dust and other solid particles.

As the gaseous stream enters the separator A it passes through the waterspray from the drums 74 and 75. The water droplets will be acceleratedin the direction of flow of the gaseous stream, however initially theirvelocity in the direction of the stream flow will be considerably slowerthan that of the incoming dust particles. Accordingly the pattern ofdroplets will provide targets for the incoming dust particles.

As the gaseous stream is directed to the venturi throat defined by theventuri rollers 35 and 36, the rapid increase in gas velocity will causethe water droplets to break down into smaller droplets and dustparticles travelling at the same velocity as the gaseous stream willovertake and impinge upon the slower moving droplets. The probabilitythat a dust particle will impinge upon a water droplet is increased asthe stream converges in the venturi throat due to the extremely highturbulence. The turbulence has the additional beneficial effect ofcausing agglomeration of dust particles by inertial impaction with thewater droplets. Also the reduced pressure in the venturi throat willcause condensation due to the vapor saturated condition of the gaseousstream. The condensation helps the particles to grow and the resultingwetness of the particles assists in their agglomeration.

The agglomerated particles leave the separator A through the outletconduit 21 and travel to the air water separator C which is ofcylindrical form. The gaseous stream enters the chamber of the separatorC in a tangential manner and takes up a helical path of travel from thebottom upwardly toward the stack 22. The centrifugal force resultingfrom the swirling of the stream as it takes up a helical path, causesthe agglomerated dust particles and water droplets to collect in anannular envelope adjacent the interior surface of the water separator Cand then to gravitate toward the bottom of the separator where thecontaminated water is drained out throu the outlet pipe 23 and directedto a clarifier 24. From the c anfier 24 about per cent of the scrubbingwater is recirculated in its clarified form back to the water troughs 7iand 72 while the remaining l0 per cent of the water containing a highconcentration of the contaminates is directed to a suitable disposalfacility such as sludge beds or the like.

While the invention has been shown and described with respect tospecific embodiments thereof this is for the purpose of illustrationrather than limitation and other variations and modifications will beapparent to those skilled in the art upon a reading of the specificationand claims. Therefore the patent is not to be limited in scope andeffect to the specific embodiment shown and described herein or in anyother manner that is inconsistent with the extent to which the progressin the art has been advanced by the invention.

lclaim:

1. Apparatus for removing solid particles from particle laden gases in agaseous stream, comprising:

a scrubber assembly interposed in said gaseous stream and having aninlet and an outlet for said stream, walls defining a passage for saidstream, said passage having a converging upstream portion and adiverging downstream portion, said walls including a pair of opposedconverging walls in said upstream portion and a pair of opposeddiverging walls in said downstream portion, each converging wall and itscorresponding diverging wall being spaced apart endwise from oneanother,

means for producing a liquid spray adjacent said inlet whereby liquiddroplets are mixed into and carried by said stream, and a pair ofrollers with spaced parallel axes journaled in said unit and locatedwith opposed cylindrical portions of the surfaces thereof positioned inopposite sides of said stream extending through the space between therespective converging and diverging walls of said passage to define aventuri throat with curved side walls that translate in the direction ofsaid stream to reduce friction, and

means for removing said water droplets and agglomerated solid particlesfrom said stream.

2. Apparatus as defined in claim 1 including at least one spray drum anda trough of water upstream of said venturi throat, said drum having abottom surface portion thereof immersed in water in said trough, meansfor maintaining a generally uniform water level in said trough and meansfor rotating said drum about its axis with sufficient surface velocitywhereby water that clings to the surface of said drum is propelledtangentially in the form of a spray of droplets into the path of saidstream.

3. Apparatus as defined in claim 2 including two of said spray drums,each having a water trough, said drums being located on opposite sidesof said stream.

4. Apparatus as defined in claim 1 including means for adjusting thespacing between said rollers whereby to change the size of said venturithroat.

5. Apparatus as defined in claim 4 wherein said rollers are journaled onaxels and including a pair of eccentric stub shafts located at the endsof each axle, each pair of stub shafts being coaxial and said axlesbeing supported in said scrubber as sembly by bearings in which saidstub shafts are pivotally sup ported, and means for turning and locatingsaid pairs of stub shafts at different angular positions about theirrespective axes whereby to change the location of said axles relative toone another.

6. Apparatus as defined in claim 5 wherein said means for turning saidstub shafts about their respective axes comprises a lever connected toone of the stub shafts of each pair.

l I II 4 I

1. Apparatus for removing solid particles from particle laden gases in agaseous stream, comprising: a scrubber assembly interposed in saidgaseous stream and having an inlet and an outlet for said stream, wallsdefining a passage for said stream, said passage having a convergingupstream portion and a diverging downstream portion, said wallsincluding a pair of opposed converging walls in said upstream portionand a pair of opposed diverging walls in said downstream portion, eachconverging wall and its corresponding diverging wall being spaced apartendwise from one another, means for producing a liquid spray adjacentsaid inlet whereby liquid droplets Are mixed into and carried by saidstream, and a pair of rollers with spaced parallel axes journaled insaid unit and located with opposed cylindrical portions of the surfacesthereof positioned in opposite sides of said stream extending throughthe space between the respective converging and diverging walls of saidpassage to define a venturi throat with curved side walls that translatein the direction of said stream to reduce friction, and means forremoving said water droplets and agglomerated solid particles from saidstream.
 2. Apparatus as defined in claim 1 including at least one spraydrum and a trough of water upstream of said venturi throat, said drumhaving a bottom surface portion thereof immersed in water in saidtrough, means for maintaining a generally uniform water level in saidtrough and means for rotating said drum about its axis with sufficientsurface velocity whereby water that clings to the surface of said drumis propelled tangentially in the form of a spray of droplets into thepath of said stream.
 3. Apparatus as defined in claim 2 including two ofsaid spray drums, each having a water trough, said drums being locatedon opposite sides of said stream.
 4. Apparatus as defined in claim 1including means for adjusting the spacing between said rollers wherebyto change the size of said venturi throat.
 5. Apparatus as defined inclaim 4 wherein said rollers are journaled on axels and including a pairof eccentric stub shafts located at the ends of each axle, each pair ofstub shafts being coaxial and said axles being supported in saidscrubber assembly by bearings in which said stub shafts are pivotallysupported, and means for turning and locating said pairs of stub shaftsat different angular positions about their respective axes whereby tochange the location of said axles relative to one another.
 6. Apparatusas defined in claim 5 wherein said means for turning said stub shaftsabout their respective axes comprises a lever connected to one of thestub shafts of each pair.